- His skills include proficiency in using various programming languages.
- His skills include proficiency at using various programming languages.
A post on Quora suggests the latter sentence is more grammatically accurate than the former, saying, "Proficient in is generally used when discussing a subject area: proficient in science, proficient in auto mechanics. Proficient at is used with a specific activity: proficient at swimming, proficient at building houses."
However, Grammarly suggests proficiency in is correct in the first sentence and that using proficiency at instead of it in that sentence would be grammatically incorrect. Which of the two sentences is grammatically correct? Are they both fine?